In 1931, a chemist named Arthur Fox sat at his DuPont Company lab bench,
mixing a powdered chemical. He accidentally let a bit of the powder blow
into the air around him. Fox and another scientist got some of the
chemical into their mouths. Dr. Fox's colleague exclaimed how bitter the
powder tasted. Fox was surprised -- he had been much closer to the
chemical, but he tasted nothing at all. Both men tasted the chemical
again. Again, Fox said the chemical was tasteless, but his co-worker
insisted it was very bitter. Fox handed out crystals of the chemical,
which were not poisonous, to his friends, family members and fellow
scientists and asked them if they tasted anything. Some people, like Fox,
tasted nothing; others found the chemical somewhat bitter or intensely
bitter.

Dr. Fox's chemical, called phenylthiocarbamide (PTC), has been used widely since its
discovery to detect genetic variation in tasting abilities. Studies soon
after the incident in Fox's lab showed that there is a genetic component
that influences how PTC tastes. Scientists found that people were much
more likely to find PTC bitter if other members of their family also found
it bitter. The evidence was so strong for a genetic link that PTC tasting
ability was used as evidence in paternity tests before DNA tests were
available.

More than 70 years later, a single gene underlying this
variation was discovered and reported by Un-kyung Kim and colleagues from
the National Institute on
Deafness and Other Communication Disorders. This was something of a
surprise, because for many years, scientists suspected that more than one
-- possibly many -- genes were responsible for PTC taste sensitivity.
Geneticists argued for years about how many genes might be responsible for
this variation and where these genes might be located on our chromosomes.

Using molecular genetics techniques with a variety of families, Kim
isolated an area on chromosome 7 that was likely to contain a gene
affecting PTC tasting ability. This region, however, also contained more
than 150 other genes. Of these, nine were known to produce proteins for
bitter taste receptors on the tongue. To narrow down their search, the
researchers figured out the DNA sequences of all nine of these genes.
They looked to see if different people had different versions of the same
gene for any of these, and, if so, if any gene variations correlated with
PTC sensitivity.

The researchers found a single gene for a bitter taste receptor that
completely explains different PTC tasting abilities. There are actually
three versions of this gene that differ from one another only slightly.
This small difference in the gene, and in the protein that it makes,
eventually forms a tongue taste receptor that has a different shape from a
"normal" bitter taste receptor.
This altered shape means that the person's receptors will not respond to
PTC and the person will not think the PTC tastes bitter. Since all people
have two copies of every gene, different combinations of the bitter taste
gene (two copies of form 1; one copy of form 1 and one of form 2; two
copies of form 3; etc.) determine whether someone finds PTC intensely
bitter, somewhat bitter, or without taste at all.

Some scientists are now interested in potential health applications of
genetic taste studies. For example, people who find PTC bitter are
suspected to find the taste of cigarettes bitter, which could make strong
tasters less likely to smoke. There are also possible correlations
between the ability to taste PTC and preferences for certain foods, which
could influence a person's diet. These types of correlations are very
uncertain, however: it's going to be a while before a phenylthiocarbamide
gene provides an excuse for why some of us think that broccoli is just
too bitter to eat.